RF Chokes for Antenna Systems and Feed Line Isolation
Find resources and guides for building and understanding RF chokes to prevent common mode currents and improve antenna performance.
Chokes are essential components in amateur radio antenna systems, designed to prevent unwanted radio frequency (RF) currents from flowing on the outside of coaxial feed lines. These common mode currents can cause a variety of problems, including RF in the shack, increased noise, and distorted antenna patterns, impacting the quality of QSOs and overall station performance. Proper choking ensures that all RF power goes into the antenna, not the feed line.
Hams often homebrew various choke designs, from simple coiled coax chokes to more effective ferrite-based solutions. Operators can find detailed explanations of how common mode chokes work, compare impedance measurements for different implementations across HF bands, and learn winding styles for toroid chokes and baluns. Resources often include practical build guides for devices like the W2DU 1:1 choke balun, helping operators reduce noise and improve their antenna projects.
Sort:
-
Notes on making the W2DU choke balun by placing several ferrite sleeves around a coaxial cable. -
How to make an effective RF Choke. The coiled coax choke is the easiest to make but also the least effective. This article includes some general guidelines for winding coax chokes on a 10cm PVC pipe using RG-58 or RG-213 coax cable. -
Essentially, a choke balun is designed to 'divorce' your antenna from the feed line. if your feed line is coaxial cable then you don't want it to be part of your antenna. you want to be able to deliver all your power to the radiator itself, i.e. 'the antenna'. a choke balun does this admirably -
In this designs G8ODE explains different methods to builds a choke balun -
Excellent article on limiting noise using chockes by Chuck Counselman, W1HIS -
Chart that present results of impedance measurements made on a variety of common-mode choke implementations across the frequency range 1MHz to 30MHz by G3TXQ -
Connecting centre fed antennas, dipoles, yagis, rhombics, loops to coaxial cable, unless care is taken, it is not difficult to end up with feeder radiation resulting in power loss and the radiation characteristics changes -
RF Choke to prevent hf currents on the feedline. This Magnetic Longwire Balun (MLB) makes it possible to efficiently use a coaxial lead-in cable with all forms of longwires, T-forms or other types of wire antennas, without the need for an antenna tuner. -
DF9CY implementation of W1HIS suggestion of inserting a common mode choke into a 40m antennas feed-line. -
RF Choke to prevent hf currents on the feedline or...1:1 Choke Balun, sometimes called the "UGLY BALUN" -
A chart presenting the results of impedance measurements made on a variety of common-mode choke implementations across the frequency range 1MHz to 30MHz by G3TXQ -
A trap on the coaxial cable, also known as choke, helps to eliminate the sneaking of the reflected RF- energy to the shack. The trap can be made from the coaxial cable that feeds the antenna -
Dispels myth about choke balun winding method -
For phased C-Poles, matching choke baluns are essential to maintain intended phasing, beam pattern, and gain. The author uses a low-loss, ferrite-core balun design with 19 turns of RG-174/U coax for optimal performance. -
Home brewing a high performance, high power common mode choke for HF bands -
This article clarifies the roles of baluns, ununs, common mode chokes, line isolators, and impedance transformers in amateur radio. A balun decouples balanced antennas from unbalanced feed lines, preventing interference. Ununs serve a similar purpose for asymmetrical antennas. Common mode chokes and line isolators suppress common mode currents, reducing noise. Impedance transformers adjust antenna impedance to match feed lines but do not decouple or suppress common mode currents. Understanding these components is crucial for optimizing antenna performance and minimizing interference. -
A general article on CMC, common mode choke include a description and tipical usage. -
Stop common-mode current when transmitting with an end-fed antenna. Unbalanced antennas are very prone to currents on the outside of the coax. -
The influence of temperature changes in Common Mode Chokes. To get an indication of the impedance behavior when the core temperature changes, the author tested a CMC by heating it with a hairdryer, and measuring its temperature with an infrared thermometer. When the CMC temperature rises from 30C to 90C, the frequency of maximal impedance shifts down from ca.10 MHz to ca.7.2 MHz, or by a factor 1.4. -
Analysis of common mode chokes using T200-2 iron powder vs. FT240-43 ferrite toroids for HF baluns. Includes VNA measurements. -
May 2015 Radcom Article on common-mode chokes that can be used for control cables, phone lines, but mostly on typical HF antenna systems. This article explain what common mode chokes does, why you meay need one, properties common mode chokes should have, how to build and measure performances. -
This article is an attempt to shed some light on this misunderstood component, covering topics like why you need a common mode coke, what it does, what properties it should have, ho to build one and how to measure its performance. -
If you want to build your own common mode choke, this article explains how to measure the efficiecy of your choke before using it. In the examples has been used Amidon FT240-31 toroids. -
How many times have you heard the advice to coil a few turns of coax at the antenna end to form a choke. How to find out if your common mode current choke really works -
A reproduction test of a a binocular ferrite core and winds a total of 7 turns is a patented rf choke with an unusual winding, although this winding scheme reduces the inductance, it might result in a much larger bandwidth over which the choke has a sufficiently large impedance. The Author test this model by reproducing the choke. -
This article presents an RF Choke featuring an 11-bifilar turn winding of #14 house wire on a Fair-rite FT240-31 toroid. The choke is enclosed in a 3D-printed case from Thingiverse, though this may pose thermal concerns at higher power levels. With SWR concerns up to 30MHz, the author plans to employ two series chokes at the rig input for improved performance. This choke offers versatility for portable use, with potential mismatch resolution using an antenna tuner. Further testing is anticipated upon the arrival of new cables. -
This is a small collection of K9YC info and my experiences. Problems by feed lines of 1/2 lambda length. CMCs in transmitting and receiving systems. Antenna unbalance, Maximal allowed power, Choke winding tips. -
W1JR-style common mode chokes are versatile tools for antenna experimentation. Three variants were constructed using RK4 ferrite cores and RG303 Teflon coax, differing only in output terminals: banana connectors for dipoles, N-connectors for antennas with existing terminals, and bolts with washers for vertical antennas. Materials included junction boxes, terminals, and small hardware. Assembly involves maximizing windings on the core, securing with ties, and gluing components. Improvements included switching to multi-stranded wire for durability. These chokes provide efficient, customizable solutions for various antenna setups.